As a fuel source substituting for fossil fuel, ethanol attracts attention, and the market size thereof is predicted to become 55 million kiloliters in the year of 2010. Ethanol used as a fuel is produced by distilling and refining a crude product obtained from a biological raw material such as corn, and dehydrating the refined product to at least 99.7% by weight.
A mixture of ethanol and water has an azeotropic point so that it cannot be concentrated exceeding 95% by weight of concentration of ethanol, which is an azeotropic composition, by the ordinary distillation method.
As a method for dehydration of ethanol, a method has been known in which a dilute ethanol aqueous solution is distilled in a distilling column so as to be concentrated to a point close to the azeotropic point of an ethanol-water system, and then an entrainer is added, and dehydration is accomplished by azeotropic distillation. However, this method has drawbacks such that it is required to have a process in which a three-component system is azeotropically distilled, and further the entrainer is recovered, so that much heat energy is required.
Another method is also available in which a plurality of molecular sieve tanks are arranged in parallel, and dehydration is accomplished while these tanks are switched over in batch mode. However, this method also has a drawback in that the regeneration of the molecular sieve tank consumes much energy.
Furthermore, use of a separation membrane that does not involve the above-described drawbacks has been proposed (Patent Document 1: JP 7-124444A).